It is known in the art to use a friction stir welding (FSW) for joining of two metal or plastic work pieces. In essence, FSW employs a non-consumable rotating tool, which interacts with the work pieces by its contacting surfaces to generate frictional heating and thus plasticizes the material in a weld zone. Local softening of the material in the weld zone facilitates mixing of material from the work pieces and upon translation of the tool along the welding direction the joint is formed. The plasticized material tends to run-off from the weld zone under process loads, which in turn might lead to formation of internal or surface breaking defects in the joint. This is circumvented by confining softened material in the weld zone with aid of the work pieces material surrounding the weld zone, the tool shoulder(s) having contact surfaces interacting with the work pieces and an appropriate support, such as backing bar, if required to hold the plasticized material in the weld zone.
For instance, JP 2007-253210, FIGS. 1-9 illustrates friction stir welding of flat metal pieces, where the confinement of the softened material is achieved by the use of two shoulders interconnected by a pin. In case of other configurations of the work pieces than flat ones, e.g. with a limited access and space for the support from the bottom of the weld zone between the inclined inner triangular walls as illustrated here, the known conventional FSW tools are not applicable. In such instances fusion welding techniques are often called upon which produce joints with inferior quality and are more generally less productive especially for thicker section welding. An alternative as illustrated in FIG. 9 of JP2007-253210 provides the opportunity to use conventional symmetrical, balanced FSW tool having the equal-sized shoulders. However a symmetrical triangular shape of the profile providing the best load force distribution through the welded panel is then not achievable.
The perfect triangulation design of the aluminum panel allows for superior structural integrity and best utilization of the structural material used. Resulting stresses from applied bending moments and forces are evenly distributed, meaning that the material is fully utilized, partly due to the fact that the forces lines of action meet within the material. Generally, triangle-shaped profile cavities dramatically increase the shear resistance compared to rectangular-shaped cavities, since no parallel sides exist in a triangle. The fixed length of the sides structural stability of a triangle assures no change of the shape upon loading, which for instance is utilized in truss structures in structural engineering. Conversely as in frame structures, a rectangular shape requires both the lengths and the angles to be fixed in order to retain its shape upon loading.
For example, this problem appears in manufacturing of so called double skin panels from multi-void hollow extrusions or profiles made of aluminum alloy material by joining extrusions with FSW method into the double skin panels, having beneficial strength-to-weight ratio. These panels might be used in building constructions such as bridges or the similar, or in a mass transportation applications such as marine, rail stock and automotive applications requiring strong lightweight constructions being resistant to corrosion. Such multi-void hollow profiles extruded from aluminum alloys usually comprise a first flat portion called plate or skin, a second flat second portion called plate or skin, and interconnecting inner perpendicular or inclined walls or trusses arranged between the first and second flat portions forming a generally triangular shape in a cross section perpendicular to the extrusion direction as shown in e.g. U.S. Pat. No. 7,665,651.
U.S. Pat. No. 7,665,651 B2 relates to butt joining of two multi-void hollow extrusions into a double skin panel by FSW. Each double skin panel comprises an upper plate and a lower plate. In order to make FSW possible, a vertical plate member is arranged between the upper and lower plates to provide support in location where the friction stir welding is performed.
The inclined inner walls of double skin panels are carrying and distributing the load through the profile providing the strength for the entire panel. The inner inclined walls of the profiles ideally form regular or symmetrical triangular shape hollows in a cross section perpendicular to the extrusion direction. The more uniform and regular the triangular shape of the profile cross sections is, the better and higher is the strength of welded final panel due to the load distribution during use of such profile. The load distribution lines through the neighboring inclined inner walls shall preferably intersect each other within the flat portions called plates or skins, i.e. within the profile or the final panel material or the panel body, in order to avoid or minimize a bending moment upon loading. Such design reduces the risk of excessive bending moments acting on panels in service and thus strengthens the entire construction.
The need of the support to the welding area from the downwards side during FSW as described above might be solved in different ways by a separate support placed under the welded area (backing bar), or alternatively as illustrated in U.S. Pat. No. 7,665,651 it might be formed by work piece extensions underneath (integral backer). This solution has the disadvantage of forming crack-like notches in the vicinity of the weld which adversely affects fatigue performance of the construction. Besides that, moisture can accumulate in the crack between the faying surfaces and lead to accelerated corrosion in the welded section, and thus weaker construction. Furthermore, such profile construction does not provide the regular symmetrical triangular shape in the cross section view of the extruded profile for the optimal exploitation of load distribution within the welded panel, which makes the panel susceptible to bending moments and shearing within some of the final welded panel areas.
A number of other solutions have been proposed, i.e. providing the support for plasticized material in the welding zone, for example as illustrated in the above mentioned JP 2007-253210, by the equal-sized shoulders attached to the pin on the both sides (so called balanced Bobbin FSW tool) or similarly to the tool of U.S. Pat. No. 6,199,745 adjusting the distance (or the pin length) between two opposite shoulders. However, the tool of JP 2007-253210 cannot be used to weld the opposite flat portion at the tapered end of the triangular space formed between the profiles to be joined, due to lack of access space for the tool between the inclined inner walls. Thus, the joint at the opposite tapered end of the triangular space must be welded by other conventional methods. This is less preferable since different welding methods lead to different properties of the welds on the two sides of the panel. The equal-sized shoulders are applying essentially symmetrical loads for plasticizing the material in the weld area and they hold the plasticized material in the joint area in place from the both sides of both welded pieces. Symmetrical loads in turn eliminate forces acting in the normal direction to the profile plates. While this is generally desirable, the solution has an inherent limitation that the above described advantageous triangulation design of inner walls that provides the best load distribution is not achievable by hitherto available equipment, because of the limited space available for an FSW tool shoulder in the desired welding zone. A tool with symmetrical shoulders small enough to fit into such limited space between the inclined triangular inner walls would have the disadvantage of providing a limited heat input for plasticizing the material, which would result in low productivity due to low process speed (or linear movement of the tool) that would not be profitable. A sufficient size of the shoulders will require enough space between the inclined walls (as illustrated in FIG. 9 of JP 2007-253210) that in turn would prevent the favorable triangular shape design of the inner inclined walls, and would preclude intersection of the force distribution lines within the panel material.
As stated above this would subject the panel to a bending moment and would thus limit the strength of the welded final product.
Some conventional tools include means to improve the heat input and thus productivity of the process. For example the shoulders can be equipped with extensions and grooves as illustrated in JP2002-263863, which together facilitate plasticized material flow. However, in this case the contact surface configuration directs the material flow from the center part to the shoulder periphery as illustrated in FIG. 5 of JP2002-263863, and thus the material might escape which can lead to a weld joint of inferior strength and increased corrosion risk.
Several methods are available for joining double skin panels, such as soldering, gluing, conventional welding, fusion welding and friction stir welding. The joint shall desirably be quickly and cost-effectively formed, strong, fatigue and corrosion resistant, without internal defects which compromise the corrosion and mechanical integrity of the structure. The flat sides of the panel should preferably be welded by the same method in order to provide the identical properties of the seams on the both sides. This is however not achievable by JP2007-253210. Additionally, the joint shall have no or little influence on the joined objects.
Another FSW tool and method is illustrated in JP2004-216435, where the upper and lower shoulders are rotated in opposite directions due to tool design which requires very complicated equipment and results in a rather poor welding seam due to intensive mixing of the plasticized material rotated by both shoulders in the opposite directions. A tool of this type cannot be used for welding limited space configurations for the reasons described above.
Joining by FSW tool and process presents extraordinary advantages in production of panels for use in different applications, as it allows manufacturing of large, homogenous panels based on extruded profiles. The FSW tool frictionally heats the work piece material locally to a plasticized state at a temperature substantially below the melting temperature. Additionally, joining of the work pieces is made without any added filling material. Thus the joined panel structure is substantially free of heat distortion. In addition, the absence of melt-related and filler induced defects known from fusion welding results in excellent mechanical properties and tightness of the welds. Also risk of intermetallic corrosion is eliminated. The rotating tool leaves a relatively smooth weld face, flush with the work piece surface. This results in longer time to crack initiation under cyclic loading. Clearly a universal FSW tool for welding of the complicated configuration extruded profiles having a limited space for the tool access at the welding zone, and process performed at the acceptable production rates and allowing welding of such work pieces independently on their configuration are needed.